1. Field of the Invention
The present invention relates generally to liquid level detectors and more specifically to liquid level detectors for use with volatile liquids in pressurized environments.
2. Description of the Prior Art
Many chemical processes today involve hazardous or volatile chemicals. These chemicals must sometimes be used at elevated pressures and in sealed containers. The sealed containers make it difficult for a human operator to determine when the supply of chemicals in the container has been depleted.
The use of volatile liquid chemicals in pressurized containers is quite common in the semiconductor industry. One process for making wafers is known as metal organic chemical vapor deposition (MOCVD). Volatile chemicals such as trimethylgallium are used. These types of chemicals must be sealed in a container to exclude air because they burst into flame upon contact with air. In addition, some of these liquids are quite toxic. A carrier gas, such as hydrogen, is introduced into the container through a dip tube and bubbled through the chemical liquid producing a vapor. This vapor is evacuated from the container and used for vapor deposition on the wafer. All of the chemical liquid in the container is eventually converted to vapor and evacuated.
In order to maintain the constant production of wafers during the process, the liquid chemical must be replaced before it is exhausted. However, the pressurized sealed containers cannot be opened and inspected in an air atmosphere. The human operators must wait until the flow of carrier gas ceases to contain any of the liquid chemical, possibly fouling a whole process run, before replacing the container.
Liquid level detectors are known in the prior art. One such example is U.S. Pat. No. 4,201,914, issued to Benno Perron. However, these prior art liquid detector systems are made for use in open air systems and are not adapted for use in sealed, pressurized environments, especially with volatile liquids.